(Not So) Stupid Shell Tricks

Graphical environments have lots of fun add-ons, such as skins, themes, and more. If you use a terminal or other shell-based applications, it's enough to make you feel left out. This month's "Power Tools" column aims to change that.

Graphical environments have lots of fun add-ons, such as skins, themes, and more. If you use a terminal or other shell-based applications, it’s enough to make you feel left out. This month’s “Power Tools” column aims to change that.

You might call this column “Stupid Shell tricks,” but that’s not quite right because we’ll see useful techniques!

For example, displaying a shell prompt in multiple colors may not do much for your resume, but knowing how to make important text appear in bright red can help you spot serious conditions. We’ll also look at shell quoting and interpretation, escape sequences, and the usual grab-bag of curiosities and oddities that are hard to find, but good to know. Let’s dig in — for fun and profit.

Prompt Preview

When a shell is ready for your next command, it prints a prompt and waits. Left-side prompts usually end with $ on Bourne-type shells like bash and ksh, with % on csh and zsh, and > on tcsh. Shells running as the superuser, root, prompt with # to remind you of your wizardly powers.

Most shells have several left-side prompts — for unfinished command lines, for debugging output, and more. The zsh and tcsh shells can also prompt on the right side. These right-side prompts are handy for periodic notices, like the “high load” warning that you’ll see later in a left-side prompt. This column won’t cover any of these right-side prompts except to say that, in general, you can set them in the same way you set the primary (left-side) prompt.

Why reconfigure your shell prompt? Well, while it’s fun and it keeps you from doing real work, you can make a prompt show information that you often want, like the current directory name or the current time. (The time can be useful if you look back to see what time you ran a particular command.) A prompt can also change dynamically — for instance, to remind you when you need to do something, to show that new users have logged in, and to warn of a high system or network load. Last but not least, you can “hijack” the prompt mechanism to do other things, like popping up new windows.

Storing the Prompt

Bourne-type shells store their primary prompt string in the shell variable PS1 (the name ends with the digit 1.) The tcsh shell uses prompt — and zsh uses either.

You can set these variables from the command line, experimenting until you get a prompt you like. Changes there will only affect the current shell invocation, though. To make your new prompt appear in all shells, you have to set the prompt string in a shell setup file in your home directory — or in the system-wide setup file, which is often somewhere under /etc. You’ll probably want to set it from a setup file that’s read by all instances of your shell — not just login shells. For bash that’s .bashrc; for tcsh it’s .cshrc or .tcshrc; for ksh it’s the filename stored in the ENV variable; and for zsh, check your manual page or just look around (there are several possibilities). In original Bourne shells, like ash, set PS1 from your .profile file and use export PS1 to pass PS1 to subshells through the environment.

Which Quote is Which?

Surrounding a string of characters with quote marks — ‘, the single quote mark, or “, the double quote — tells the shell to disable the special meaning of some or all of the characters between the quotes. In a nutshell, that’s what quoting does!

An opening single quote disables all special characters until the next single quote character is found. (There is one exception: in tcsh, single quotes don’t disable !, the history substitution character.) Double quotes match in the same way as single quotes, but they allow most substitutions: variables (like $USER), commands (like `date` or $(date), and so on. Both types of quotes disable the word-separator characters SPACE, TAB and NEWLINE — which keeps the shell from breaking the quoted string into separate arguments. (That’s important when you set a prompt because a prompt must be a single long string, even if it contains spaces or newlines.) Let’s see some quoting examples using tcsh:

The last few examples show that double quotes can surround single quotes (and disable their special meaning), but single quotes can’t surround other single quotes. Why? After the first quote is found, the next quote of the same kind, reading from left to right, makes a pair. (In other words: quotes of the same type do not nest.)

The last example passes a literal single quote character (in the word It’s) to the echo command, while also surrounding `date` and $USER with single quote characters to disable the ` and $ characters. How? We surrounded the first three characters (It’) with double quotes and the rest with single quotes. Also notice that there are no unquoted spaces; this doesn’t matter to echo, but it’s important when you set a prompt.

These terse examples show almost everything about quoting. The rest is practice, checking the rules (from this sidebar) and trying more examples.

Setting the Prompt

The simplest prompt is just a character — with a single space afterward to make the command line stand out. The prompt must be a single string, so put its value inside quotes to keep the shell from breaking the string at spaces. The sidebar “Which Quote is Which?” explains what kind of quote to use.

Let’s start with a basic prompt: the current directory (from the shell variable PWD or cwd) and a $ or a >. These examples seem to work at first, but they have a problem that you’ll see as you change directories around the filesystem:

The current directory in the prompt isn’t changing. Why? It’s because the prompt string was surrounded with double quotes. As the sidebar “Which Quote is Which?” explains, the shell expands shell variables inside double quotes before executing the command line. So the prompt contains the value of $PWD as it was at the time the PS1 or prompt variable was stored! (To confirm this, use the command set to see current settings of all shell variables. Its output will show that PS1 and prompt both contain a literal string like /home/jpeek.)

How can we fix that? Some shells, like bash, re-evaluate the prompt string before emitting each prompt. In those shells, you can store special characters (like $ and ‘) inside the prompt string, and each one is expanded before each prompt. As the sidebar below shows, you can preserve all special characters by putting them inside single quotes:

But tcsh and csh would prompt with a literal “$cwd >” because they don’t re-interpret the prompt variable (and expand special characters like $ and `) before emitting each prompt.

The standard fix is to define an alias named something like setprompt that sets the prompt variable to a literal string. Then, each time you want to change the text in the prompt — say, after cd, pushd, or popd — call the setprompt alias. You could define aliases like these:

You can also use the tcsh variable named precmd to reset the prompt variable before each prompt, as we’ll see later. But luckily, there’s an easier way to handle most cases.

You usually don’t need to go through these quoting and alias nightmares because most shells also understand special format strings that you can store in the prompt string. Those strings expand into common things you’d want to put in your prompt, like the current directory. Your shell’s manual page should list its format strings.

Using Prompt Format Strings

A basic prompt might simply show the current directory, but the same techniques can also make prompts with a lot of information. Adding lots of stuff to a prompt can take a lot of room across the screen, leaving less room for command lines. You can solve that with a multiline prompt that puts extra stuff on separate lines, leaving the final line almost empty.

So let’s go wild and make a fairly complex three-line prompt. The pattern is shown below:

username@hostname time directoryhistorynumber X

The prompt starts with a blank line to separate it from the previous command’s output. The X should be $ on bash, > on tcsh, and # on any shell run as root. For example:

jpeek@kumquat 15:39:36 ~/articles
103$ cd /usr/local

jpeek@kumquat 15:40:01 /usr/local
104$

That’s simple to program on bash. This does the trick:

PS1=’\n\u@\h \t \w\n\!\$ ‘

The first \n in PS1 makes a leading blank line, and the second \n separates the two lines of text. The final \$ outputs $ for non-superusers or # if you’re root.

It’s trickier in tcsh because you have to embed a newline character in the prompt variable — which, in tcsh, requires a backslash (\) before the literal newline:

set prompt = ‘\
%n@%m %P %~\
%h%# ‘

Z Shell syntax looks a lot like tcsh — although, as usual, zsh has many more features. The Korn shell doesn’t support these prompt format strings, but you can simulate them by using shell and environment variables. For a little shell programming adventure, Listing One shows the example shown immediately above rewritten for ksh.

Because ksh evaluates the stored prompt string (which is set at the end of the code), and PS1 contains a call to the shortdir() function (which is defined earlier), that means shortdir() is called each time a prompt is printed. shortdir() outputs the current directory pathname; if you’re under your home directory, its name is replaced by a ~ character. shortdir() also uses the ksh operator ${variable#pattern}, which edits the value of $PWD (the absolute pathname of the current directory) to remove the leading value of $HOME and the trailing slash (/).

As we store the prompt string, the first and last parts are inside double quotes, so the values of ($USER, $hostbase, and $promptend) are expanded as the prompt is stored. The rest of the prompt — the calls to date and shortdir(), and the history number ! — isn’t interpreted until just before each prompt is printed. (Instead of surrounding some parts with double quotes, you can simply put single quotes around everything — and ksh would interpret all special characters before every prompt. But there are times when you want to set a particular value once and not have it re-interpreted before every prompt; that’s where this quote-switching technique is useful.) You may notice that ksh doesn’t require a backslash before literal newlines inside quotes, and calling date +%T makes date output only the time of day.

To keep things simple from here on, we’ll only show bash. For other shells, please compare the bash examples to your shell’s man page — and use aliases like setprompt, or setups like Listing One, as needed.

Beyond Prompt Format Strings

If you’re new to shells, the Korn shell example at the end of the previous section may seem to stretch the limits of what’s reasonable. But a shell is actually a programming language interpreter, where the language is Linux command lines. There’s no reason not to use the shell’s programmability on its interactive features — like prompts. So let’s keep on digging!

Built-in format strings can’t put everything you might want into a prompt. For instance, do you use the shell’s directory stack (described in the Power Tip at the end of the February, 2003 column, available online at http://www.linux-mag.com/2003-02/power_01.html)? You might want to put the stack list in your prompt, so you don’t have to check it constantly. That’s as simple as replacing the \w in the bash prompt string with $(dirs), which uses command substitution (the $(cmd) operator) to run the dirs command and replace the command with its output:

PS1=’\n\u@\h \t $(dirs)\n\!\$ ‘

Let’s make another version where each item on the stack is numbered. (This is especially handy for commands like pushd +2, which changes to the second directory on the stack.) To simplify things a little, let’s get rid of the time — which gives prompts that look like this:

jpeek@kumquat 0=/usr/proj/foo 1=~ 2=~/bin
107$

Even if you don’t want a prompt like that, the way to make it is worth a look! See Listing Two. We’ve changed PS1 by calling the do_dirs function instead of the dirs command (and, as we said, removing the \t that gave the current time). The do_dirs() function calls dirs with the -v option, which outputs a numbered list of directories in the stack, one per line. We pipe that output to a while loop. The loop-controlling read command reads the dirs -v output, line by line from the pipe, assigning the number to the shell variable n and the directory path to the variable dir. In the body of the loop, for each directory, we output a space character, followed by the number and the path, with an equal sign between. The -n option tells echo not to output a newline — so the entire output of do_dirs() is a single line, like 0=/usr/proj/foo 1=~ 2=~/bin, which appears in the middle of the prompt string.

You might wonder why we used a semi-bizarre shell function instead of, say, writing this in Perl. The main reason is that every command we used in Listing Two is built in to the shell, which means the prompt will be set almost instantaneously — even on a busy machine. Starting a new process (for Perl) before each prompt could make your shell seem sluggish.

The shell function also uses a while loop with its standard input redirected. This is a very handy technique in Bourne-type shells. As the bash manpage explains, while is a compound command. Most compound commands can be preceded with a “feeder” command and a pipe, as we’ve done in do_ dirs(). That means all commands within the compound command will take their standard input from the pipe. In our while loop, the only command that reads its stdin is the read command. Each time we call read, it reads a single line from its standard input.

So this while loop reads the dirs -v output line-by-line.

Another way to redirect the input of a compound command is with the shell’s < (less-than) operator, which redirects stdin from a file. Put that operator, followed by a filename, at the end — in a while loop, that’s just after the word done:

while read variables
…
done < filename

Escape Sequences, Pre-prompt Commands

You can also put color, boldface/bright, or blinking text in your prompt. For instance, you could add the word root in reverse video to the superuser prompt.

Your shell needs to know how wide the prompt is — so, for instance, you can’t “backspace over” the prompt while you edit the command line. Because escape sequences don’t output visible characters (because they don’t take room on the screen), you need to tell the shell not to “count” those characters. In bash, do that by surrounding each escape sequence with \[ and \]. In zsh and tcsh, surround an escape sequence with %{ and %} (though you don’t need to do this with zsh/ tcsh operators like %S that set standout, boldfacing, and underlining modes in the prompt).

Maybe you have accounts on multiple workstations. One of them — the master server — is special. You want to make that host have a special prompt with the hostname in reverse video. You could put the following code into your .bashrc file.

If the hostname is server, we set the special PS1 string. It starts with a “start escape sequence” marker, then an ESC character (which, in a bash version 2 prompt, is written \e), then the reverse-video escape sequence and the “end escape sequence” marker. After the hostname (which bash expands from \h), we use another escape sequence to cancel reverse video. The prompt ends with \$, as explained earlier.

Here’s another escape-sequence example. You might want part of your prompt to be highlighted when something happens. Listing Three (pg. 37) shows how to add the red words HIGH LOAD, and the one-minute load average, to your bash prompt when the system load average is 5 or more.

The setprompt() function declares two local variables (used only within the function). Next it reads the system’s one-minute load average from /proc/loadavg into the variable named load. (The rest of the contents are read into the variable etc, which we don’t use — but is required so read will split the input line.) The bash operator ${variable%pattern} strips the string matched by pattern from the end of the shell variable. Here we’re removing the dot, and everything after it, from the load average; this removes the decimal point and digits to leave an integer. The script tests the integer and if it’s greater than or equal to 5, it sets a prompt that has the red words HIGH LOAD, plus the complete load average number, at the end of its second line. If the load average is under 5, the code sets a normal prompt.

By storing the name of this function in the bash PROMPT_ COMMAND variable, the function runs before each shell prompt is printed. (In zsh and tcsh, you’d use the precmd variable instead.)

Although it’s called PROMPT_COMMAND, that variable doesn’t need to be used only to set the prompt. It can run any command you want (though, of course, you should use discretion: do you really want to rebuild the kernel before every prompt?). Listing Four shows a shell function named wrister() that reminds you every ten minutes to give your wrists a break from typing. (This is worthwhile only if you’re using the shell a lot; the time interval is only checked before a shell prompt is printed.) The function remembers the value of the bash variable SECONDS, which counts up from 0 each time you start a shell running, and compares it to the current SECONDS. If you’re using X (if the DISPLAY environment variable is set), it pops up an xmessage window; otherwise, it rings the terminal bell, prints a message in blue and waits for you to press the RETURN key.

A Linux daemon is a process that runs on its own — typically detached from any shell, as we saw in the final xmessage example. You can start daemons from one of the system /etc/rc* files that are read when the system reboots — assuming you have superuser access, that is. But that won’t restart daemons that exit for some other reason — such as exhaustion — after your Linux system has gone a year between reboots.

An easy answer is a cron job that tries to restart a daemon every so often, like once an hour or once a day. (If the system has just rebooted, the cron job will start the daemon.) This trick depends on the daemon having a “sanity check” to prevent itself from running twice: a lockfile, scanning ps output for its name and/or the PID number saved the last time it started, etc.

The && operator aborts if cd fails. The exec replaces cron’s shell process with the setiathome process, which means the shell doesn’t have to hang around, doing nothing except using some memory and a slot in the system’s process table. Some shells will do the exec automatically when they see there are no more commands to run. You can find out what your shell does by omitting the exec, then running ps lx after cron starts the daemon; see whether the daemon has a PPID (parent process ID) of 1. If the PPID is 1 (the system’s init process), there’s no shell waiting for the daemon to finish.

Jerry Peek is a freelance writer and instructor who has used Unix and Linux for over 20 years. He’s happy to hear from readers at jpeek@jpeek.com.
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